It is known that a number of factors affect the efficiency of an alternating current (AC) motor drive. (As used herein, the term "drive" is intended to include the motor, its power supply, and the control of the power supply, including command and feedback signals.) One of the main factors is that the closer the wave shape of the power supplied to the motor approaches a sine wave, the lower the total harmonic distortion. Lower harmonic distortion means less motor heating and improved efficiency.
When a variable speed AC drive is desired, it is common to effect that variable speed by furnishing the motor with variable frequency power. This is customarily achieved by furnishing power to the motor by way of a power conversion bridge, the most common form of which is a three phase bridge comprised of three legs, each having a positive side and a negative side in what is often referred to as a six step inverter. Since each of the legs of the inverter is normally formed of a semiconductor switch arrangement operating in a binary mode, the basic output of the inverter is essentially a three phase, square wave line-to-line voltage. One way of improving the wave shape of the inverter output, that is to better approach a sine wave, is through the use of pulse-width-modulation (PWM). In a PWM inverter the basic output of the bridge is a square wave having a fundamental frequency which largely determines the motor speed. PWM places pulses (or notches depending upon the way the wave shape is viewed) within the basic square wave output of the bridge to improve the wave form; i.e., to make it more like a sine wave, and therefore reduce the harmonics distortion. As a general rule, the greater number of pulses, the better the wave form.
Increasing the number of the pulses is, however, not without problems since there are losses associated with each switching of the semiconductor switches of the bridge. These losses occur both when the switches are turned on and turned off and are a function of the current being carried. As such, the higher the frequency of switching and the higher the current carried by the switches, the higher the losses.
One example of a situation in which efficiency is extremely important is that of on-road electric vehicles. As is known, the major problem with these vehicles today is the energy-to-weight ratio of the battery systems which results in a serious restriction on the range of the vehicle. It is, however, important to realize that in this particular application, greater overall efficiency can be realized by maximizing the efficiency during the times of lighter loads since this is where the vehicle operates the largest amount of time. For example, in what is considered to be a typical application, a vehicle might require approximately 5 to 10 pound-feet of torque when cruising (e.g. in the 50 to 60 miles per hour range), the mode of operation most common from a time standpoint. Heavy torque requirements, those above 15 pound-feet are normally of a very transient nature, such as upon starting or climbing a steep hill. Thus, enhanced overall efficiency can be achieved by optimizing the efficiency of the operation of the drive during the longer periods of operation which are at lighter torque requirements.